Since a liquid crystal display element included in a liquid crystal display apparatus does not emit light for itself, a backlight device must be provided on the back surface side of the liquid crystal display element as a light source for illuminating the liquid crystal display element. In recent years, demand for thinner liquid crystal display apparatuses has been rising. Sidelight-type backlight devices which include a thin plate-like light guide plate and a light source disposed so as to face a side of the light guide plate and create a planar light source by entering light from a side face of the light guide plate are widely used.
The conventional mainstream of the light source of the backlight device was cold cathode fluorescent lamp (CCFL), which produces white light with a fluorescent substance applied on an inner wall of a glass tube. However, since performances of light emitting diodes (LEDs) have been tremendously improved in recent years, demand for backlight devices using LEDs as light sources is rapidly increasing.
Devices known as LEDs can be divided broadly into two types. A type of an LED is a monochromatic LED, which produces monochromatic light of red color, green color, blue color, or the like by direct emission of the LED. Another type of an LED is a multicolor LED, which includes a monochromatic LED and a fluorescent substance and produces light of a plurality of colors by exciting the fluorescent substance by the light emitted from the monochromatic LED. Some multicolor LEDs include a monochromatic blue LED and a fluorescent substance which absorbs blue light and produces light of green color to red color. The multicolor LED can configure a white LED which generates white light having a wide spectrum ranging from the blue color to the red color. Since the white LED has high emission efficiency and is effective for reducing power consumption, it has been widely used as a light source of the backlight device.
A Liquid crystal display element in the liquid crystal display apparatus includes color filters. The liquid crystal display apparatus displays colors using wavelengths of red color, green color and blue color filtered by the color filters. If the light source has a broad wavelength bandwidth of a continuous spectrum, like the white LED, in order to expand the color reproduction range, the liquid crystal display apparatus must improve chromatic purity of display colors by narrowing the wavelength band of light passing through the color filters. Narrowing the wavelength of light that passes through the color filters increases the amount of unwanted light. In other words, usage efficiency of light by the liquid crystal display element deteriorates greatly. Accordingly, the luminance of the display surface of the liquid crystal display element lowers. Furthermore, if it is tried to improve the luminance, the power consumption of the liquid crystal display apparatus increases.
The CCFL and white LED in general use have a peak at about 615 nm (a wavelength obtained by shifting a wavelength from red color toward orange color) in the red wavelength range because of the characteristics of the fluorescent substance. If it is tried to improve the chromatic purity in the wavelength region of 630 to 640 nm, which is preferable as pure red in the red color, the amount of transmitted light decreases extremely, thereby lowering the luminance significantly. The CCFL and white LED have a small amount of energy especially in the spectrum of the red color of 600 nm to 700 nm. If it is tried to improve the chromatic purity in the wavelength region of 630 to 640 nm, which is preferable as pure red in the red color, the amount of transmitted light decreases extremely, thereby lowering the luminance significantly.
In order to expand the color reproduction range while minimizing loss of light by the color filters, alight source that emits a light ray of a narrow wavelength bandwidth must be used. That is, in order to expand the color reproduction range, a light source that emits a light ray with a high chromatic purity must be used. In recent years, a liquid crystal display apparatus using monochromatic LEDs that emit light rays of the three primary colors, red, green, and blue, respectively is proposed. A liquid crystal display apparatus using monochromatic lasers that emit light rays of the three primary colors respectively is also proposed. In these liquid crystal display apparatuses, the three primary colors of light are mixed to generate white light.
In recent years, a liquid crystal display apparatus including a backlight unit using monochromatic LEDs or lasers with a narrow spectral width as the light source are proposed to expand the color reproduction range. The narrow spectral width means that the chromatic purity is high. Particularly, a laser has a very high monochromaticity. A laser also has high light emission efficiency. Accordingly, use of LEDs and lasers makes it possible to provide bright images with a wide color reproduction range. Use of LEDs and lasers also makes it possible to provide a low-power-consumption liquid crystal display apparatus. Particularly, because of their high monochromaticity, lasers can expand the color reproduction range greatly and can improve the picture quality of the liquid crystal display apparatus significantly.
When the light rays emitted from these light sources are mixed to generate white light, however, a difference in spatial luminance distribution on the display surface of the liquid crystal display element of the different colors appear as color unevenness. To reduce the color unevenness, the spatial luminance distributions in the surfaces of different colors must be improved in uniformity. However, light emitted from light sources using different light emission principles or light emitting elements of different properties differ in divergence angle or light emission efficiency, and the number of light sources to be disposed and the ways of disposing the light sources vary. For these reasons, an optimum means for uniforming the spatial luminance distribution in the surface must be provided for each light source.
If point light sources such as LEDs and lasers are used as light sources of a sidelight-type backlight device, luminance in the vicinity of the light source is considerably high. This results in unevenness in luminance around the incident end of light. That kind of unevenness in luminance can be reduced, for example, by arranging a great number of point light sources at short intervals in a row analogously to a linear light source. Since the backlight device of the liquid crystal display apparatus requires high uniformity in spatial luminance distribution in a surface, a quite number of light sources would be necessary. This would increase the power consumption, lower the manufacturability, and increase the cost.
If the light rays of different colors have different unevenness in luminance, color unevenness occurs. Unevenness in luminance and unevenness in color in the liquid crystal display apparatus will decrease picture quality remarkably. Therefore, the backlight device is required to provide planar light of each color with a uniform spatial luminance distribution in the surface. The spatial luminance distribution in the surface is a distribution indicating the degree of luminance in positions expressed two-dimensionally in a given plane.
A technology for uniforming the spatial luminance distribution in the surface with a minimum number of light sources when LEDs and other point light sources are used has been reported. For example, in the technique disclosed in patent document 1, the light emitting elements in the liquid crystal display apparatus are covered with hemispherical transparent materials including a plurality of materials having different refractive indices. The light rays emitted from the light emitting elements are diffused by refraction, and the distribution of light in the light input portion of the light guide plate can be made closer to light having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source). The linear light source is a light source that emits a light ray having an almost uniform spatial luminance distribution in one-dimensional direction. The light rays from a plurality of point light sources can be overlapped to generate a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source).
For example, a planar light source unit in patent document 2 has a light diffusing face in the back surface of a light guide plate, and the light diffusing face is provided to convert a light ray from a point light source to a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source). In addition, the planar light source unit has a light diffusing face for uniforming the spatial luminance distribution in the surface of the backlight device. In the diffusing face for converting a light ray from a point light source to a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source), the rate of covering the diffusion material is lowered in a part where the point light source has a high luminance. In a part where the point light source has a low luminance, the rate of covering the diffusion material is increased. With this configuration, a light ray from a point light source can be converted to a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source).
Backlight units including a special light guide plate for each light source having different properties for the purpose of suppressing color unevenness is also reported. For example, a backlight unit for flat display panel including a special light guide plate for the light source of each color is proposed in patent document 3. This backlight unit for flat display panel includes different light sources of different colors and light guide plates corresponding to the light source, and they are layered. In this backlight unit, monochromatic planar light rays exiting the light guide plates are added up to generate a white illuminating light ray. With this configuration, the structure of each light guide plate can be optimized for the properties of the single light source corresponding to the light guide plate. Accordingly, this configuration allows the uniformity of the planar spatial luminance distribution of each color to be improved, consequently color unevenness to be suppressed. The planar spatial luminance distribution is a distribution indicating the degree of luminance with respect to a position expressed in two dimensionally in a given plane.
The technology according to patent document 1 and patent document 2 adds an optical element for converting a light ray from a point light source to a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source). Accordingly, a light ray having an approximately uniform spatial luminance distribution in a one-dimensional direction can be sent into the light guide plate from its face. Then, a linear light source having uniform planar spatial luminance distribution can be obtained. These optical elements, however, require complicated structures. If a point light source with a high directionality, like a laser, is used as a light source, a complicated optical element with higher diffuseness would be required. That type of optical element would require a longer optical distance to convert alight ray from the point light source to a light ray having a linear shape in the cross section perpendicular to the light traveling direction (a light ray similar to a light ray emitted from a linear light source), so the apparatus would become large. Therefore, these types of optical elements are not suitable when a laser light source is used.
A backlight unit based on a technology disclosed in patent document 3 includes a light guide plate for each of a plurality of light sources having different properties. Accordingly, a planar light source having suppressed color unevenness and uniform planar spatial luminance distribution can be obtained. In that configuration, a plurality of light guide plates must be layered, and the backlight unit would become large especially in the direction of thickness of the unit.